1. Field of the Invention
The present invention relates to polymeric compositions and their use for blocking subterranean formations in oil and gas fields.
2. Discussion of Realted Art
Oil and gas fields produce water as well as oil and/or gas, especially when the well is depleted. In addition. secondary recovery techniques such as water flooding to stimulate production of oil involve injection of water under pressure at a distance from a production well to squeeze the oil out. However, in both cases the water moves in the formation along least hindered paths, so that the recovery technique may be inefficient, and in the direct recovery increased proportions of water are produced.
To enhance reservoir conformance control, i.e. to mobilise the oil that may be present in less permeable areas, blocking agents may be injected to obstruct the high permeability channels thereby encouraging preference for liquid movement via the lower permeability channels. Among known blocking agents are polymer gels, in particular gels of polyacrylic acid or polyacrylamide/polyacrylic copolymers, cross-linked with chromium ions as disclosed, for example, in U.S. Pat. Nos. 4,744,418 and 4,844,168. The copolymer, mixed with cross-linker, is injected into the formation from the production well, often after a flood of cold water to pre-cool the formation rock, to stop premature cross-linking and gelling before the mixture reaches its desired position. Much work has been described to reduce the rate of cross-linking, by reducing the activity of the cross-linking metal ion, e.g. by co-ordinating the chromium with a ligand, e.g. lactate. Other cross-linking agents which have been used are metals such as zirconium and other transition metals.
Whilst the use of metal cross-linking agents is reasonably satisfactory in practice, there are certain disadvantages. For example, there can be environmental effects with the use of metal ions, particularly with chromium. For some rocks, such as carbonate rocks, the metals used as cross-linking agents are absorbed by the rocks and are hydraulically unstable at temperatures above 70.degree. C. and so cannot function properly, thus reducing the effectiveness of the blocking polymers.
U.S. Pat. No. 4,773,481 describes a process for reducing the permeability of a subterranean formation by the gelation of water-soluble polymers of polyalkyleneimines and polyalkylenepolyamines with certain polymeis which are anionic or hydrolysable to form anionic polymers and which are cross-linked by the water-soluble polymers. Examples of the anionic or non-ionic polymers are polyacrylamide and alkylpolvacrylamides, copolymers of polyacrylamide and alkylpolyacrylamides with ethylene, propylene and styrene, polymaleic anhydride and polymethacrylate and hydrolysis products thereof. As described in the patent, when the water-soluble polymer and the anionic polymer are mixed, a viscous gel is quickly formed. In use, a solution of the water-soluble polymer is pumped into the subterranean formation first, followed by water to cleanse the bore of the water-soluble polymer to prevent premature gelling upon introduction of the anionic polymer. and finally the anionic polymer is introduced.
This three step procedure has a number of disadvantages in practice but it is necessary because the water-soluble polyalkyleneimine or polyalkylenepolyamine reacts very quickly with the anionic polymer and hence cannot be premixed before injection without substantial risk of premature gelation. To some extent, this problem can be reduced by using a non-ionic polymer which is hydrolysable to an anionic polymer at a relatively slow rate, but this limits the choice of materials and is not generally satisfactory. So far as we are aware, the process of U.S. Pat. No. 4,773,481 has either not been commercially utilised, or has only been used to a small extent, presumably because of the substantial practical difficulties involved.